1. Field of the Invention
The present invention relates to novel biaxially drawn polyester films containing fillers, which have improved surface characteristics and properties, and also to the use of such films, especially in the manufacture of magnetic tapes or capacitors therefrom.
2. Description of the Prior Art
In order to be advantageously used as base substrates for magnetic tapes, polyester films must satisfy two requirements in particular, which in practice are ofttimes competing or contradictory. On the one hand, these films must have a good slip, enabling them to be used in industry, and, in particular, to be rolled up, and on the other hand, the production of high quality magnetic tapes requires films having face surfaces which are as smooth as possible, such that defects in the reproduction of signals which are to be recorded thereon are avoided to the greatest possible extent.
One solution frequently proposed for improving the slip of the films by reducing their coefficients of friction consists of incorporating inert, typically inorganic fillers into the polyester. Thus, kaolinite, silica and other inorganic metal oxides or salts are commonly used for this purpose. However, the presence of these fillers on the face surfaces of the films results in disadvantages at the magnetic tape stage. This is all the more troublesome because attempts are currently being made, in particular in the field of video tapes, to produce magnetic tapes having a face surface with the minimum possible number of defects. This quite obviously requires the base films to themselves have excellent surface characteristics and, notably, an increasingly low degree of roughness consistent with the quality required of the desired finished products.
The surface roughness of a film is typically expressed with reference to two particular values:
(i) a value corresponding to the total or maximum roughness R.sub.t (also called PV, representing the expression "peak-to-valley"), which is defined as the maximum depth of the roughness; and
(ii) a value corresponding to the arithmetic average roughness R.sub.a (also called CLA, representing the expression "center line average").
The total or maximum roughness R.sub.t and the average roughness R.sub.a are defined and their measurement is described in Japanese Industrial Standard JIS B 0601-1982.
Other fillers are also used, such as, for example, titanium dioxide; the latter affords a better compromise between the coefficient of friction and the roughness, but the substantially haze which it imparts to the film makes it difficult to recover the product waste, although this recovery is economically necessary.
It has also been proposed, in Japanese Patent Application No. 87,936/1975 published under No. 40,929/1980, to use inert fillers whose particles are essentially distributed in two populations of different sizes; some of the particles have a size of 3 to 6 microns and other particles have a size of 1 to 2.5 microns.